It's a cold day in Midwest America. I was suffering from the flu after 3 days in Yellowstone. It is a stunningly raw and beautiful park in the winter, but it can also be extremely cold. The steam from the thermal activity freezes on whatever it can - including the big bisons that gravitate there to keep warm. I have waited for a very cold day like this in Yellowstone for a long time, but looking at me that morning I certainly paid the price.

This kind of photography may test one's appetite to work in extreme conditions, but it also tests our maths. I have never taken a photograph of anything on an aperture of F18 before, but I did for this image as I wanted as much depth of focus as possible. My inclination was to be greedy and to try and have the bison’s nose in focus and also her eyes. So I sacrificed shutter speed and resolution and hoped for the best. It was impossible to check too much in the field, but when we returned home, I found one gem.